During the continuous casting of steel, and particularly aluminum-killed steel, it is desirable to prevent oxidation of the liquid metal being cast. Constituents of the melt, such as iron, aluminum, manganese, chromium, titanium and the like, are converted to oxide when exposed to air. These oxides are a source of nonmetallic inclusions in the solidified metal and can lead to serious defects. At the same time, the loss of these constituents through oxidative processes changes the composition of the resultant alloy.
To prevent such oxidation of the molten metal, fluxes are added to the mold during the continuous casting of steel. These fluxes melt to form a protective layer of liquid flux over the surface of the molten steel in the mold, thereby excluding atmospheric oxygen. In addition, this melt lubricates the walls of the mold, reducing friction between the mold and the solidifying strand of steel as it is continuously withdrawn. The liquid flux also serves to dissolve any oxides introduced into the mold with the liquid metal, thereby allowing for the production of clean, inclusion-free steel.
Characteristic of these types of fluxes are those described in U.S. Pat. Nos. 3,649,249; 3,704,744; 3,899,324; 3,926,246; 4,092,159 and 4,204,864; and U.S. application Serial Number 26,925 filed April 4, 1979, the disclosures of all of which are herein incorporated by reference.
When aluminum-killed steel is being processed, considerable amounts of aluminum oxide are dissolved into the flux. Even though only 0.04 percent aluminum may be in the steel itself, the aluminum oxide in the flux increases very significantly. Thus, a flux with no initial alumina can increase to an alumina content of 25 percent with a considerable increase in viscosity in the liquid flux. In fact, the flux may become so viscous that it no longer performs its functions of dissolving nonmetallic inclusions and of lubricating the strand being withdrawn from the mold.
When this occurs, a hole in the shell of the strand may form which would allow all of the liquid metal in the mold to escape (this is referred to as "breakout"). This problem is serious enough to cause termination of the cast and necessarily requires considerable time to return the machine to working order. Currently, there are two alternatives to avoid this problem. One option is to stop the machine and remove all molten slag or flux in the mold using skimmers. Fresh mold powder is then added and the cast is restarted. This option is, of course, somewhat hazardous and may additionally lead to scratching of the soft metallic (usually copper) lining in the mold. Such an option also exposes the metal to oxidation from the air. Finally, the portion of the cast formed during the period when the machine is stopped may well have to be discarded.
The second option is to add to the mold a very aggressive material that has high solubility for alumina and thins down the various flux. If very carefully done, this expedient may allow the cast to continue since the old flux would be removed from the mold as a lubricating film with the exiting strand. However, this technique requires good mixing to occur in the mold. Frequently, the viscous flux does not react quickly enough with the additives. The additives will then melt to a liquid that is too fluid to maintain a lubricating film and a breakout will then occur.